FIG. 1 illustrates a known (prior art) building roof structure R comprising a corrugated metal or other roof deck D supported on plurality of parallel, spaced-apart purlins, trusses or other structural members P that extend axially along respective longitudinal axes PX that each extend perpendicularly into and out of the page as shown in FIG. 1. Between each successive pair of purlins P, an open bay B is defined, and the roof deck D spans the bays B. It is known to insulate the roof deck D with a known roof insulation system 10. Typically, as shown in FIG. 1, the known roof insulation system 10 comprises a vapor retarder facing or web or sheet S draped over the respective upper flanges or edges P1 of multiple (at least two) successive purlins P so that the sheet S spans one or more bays B. The vapor retarder sheet S may be fixed to at least some or all of the purlins P over which it is draped using suitable fasteners, adhesive or other means (the vapor retarder sheet S is usually secured to the first and last purlins P over which it is draped). The vapor retarder sheet S is a single layer or multiple layer product, e.g., single-layer vinyl film/sheet or other film/sheet, or a laminated composite containing various combinations of aluminum foil, polymeric film/sheet, kraft paper, reinforcing yarns and fabrics. Vapor retarder sheets S vary in strength, color, light reflectivity, and their ability to retard moisture migration therethrough. An insulation space SP is thus defined between the inner face S1 of the sheet S and the roof deck D (i.e., the sheet inner face S1 is oriented toward the roof deck D), and fiber glass or other insulation I is laid or blown or otherwise installed in the insulation space SP and is supported on the inner face S1 of the vapor retarder sheet S and/or laminated to the inner face S1 vapor retarder sheet S. As noted, the vapor retarder sheet S inhibits migration of moisture into the insulation and improves aesthetics of the interior of the building.
FIG. 2 is identical to FIG. 1, except that a drawback of the system 10 of FIG. 1 is illustrated. In particular, during installation or of the vapor retarder sheet S, the sheet can be pulled too tightly (over-tensioned) across one or more bays B, in a direction transverse to the purlin longitudinal axes PX, so that the height (relative to the roof deck D) and volume of the insulation space SP is diminished, leading to a reduction in the available space for insulation I and/or leading to undesired compression of any previously installed insulation I, both of which reduce the efficiency or “R-value” of the insulation I. Another, related deficiency of the known roof insulation system 10 is that the vapor retarder sheet S might not pulled tight enough (under-tensioned) across one or more bays B, which leads to a sagging appearance and/or can cause the insulation I to move away from the purlins P toward the middle of the bay B, leaving the lateral areas of each bay B adjacent the purlins P under-insulated.